Method of manufacturing a sintered oxidic ferromagnetic body

ABSTRACT

Stringent requirements as regards the resistance to detrition are imposed upon the sintered oxidic ferromagnetic material of which particularly pole shoes of magnetic heads are manufactured, when said heads are used at very high tape speeds. No crumbling away of crystals or groups of crystals must take place. Sintered oxidic ferromagnetic material which consists of large interlocking crystals (diameter more than 50 microns) has a high resistance to detrition. A method of obtaining such a crystal structure comprises the addition of from 0.01 to 0.1 percent by weight of an alkaline earth metaborate to the starting mixture of the metal oxides to be sintered, e.g. NiO, ZnO and Fe2O3.

United States Patent Peloschek et al. 5] June 20, 1972 [54] METHOD OF MANUFACTURING A 3,472,780 10/1969 Stuijts et al. ..252/62.62

SINTERED XIDIC FERR MAGNETI BODY O 0 C FOREIGN PATENTS OR APPLICATIONS 1,094,995 7/1962 Germany [72] ts; 32:2 :32? 820,855 9/1959 Great Britain ..252/62.58

Y 9 Netherlands Primary ExaminerRobert D. Edmonds [73] Assignee: U.S. Philips Corporation, New York, NY. Attorney-Frank R. Trifari [22] Filed: March 11, 1970 [57] ABSTRACT [21] Appl' l8676 Stringent requirements as regards the resistance to detrition are imposed upon the sintered oxidic ferromagnetic material [30] Foreign Application Priority Data of which particularly pole shoes of magnetic heads are manufactured, when said heads are used at very high tape speeds. March 20, 1969 Netherlands ..6904352 N0 crumbling away of crystals or groups of crystals must take place. Sintered oxidic ferromagnetic material which consists if fiii of large interlocking crystals (diameter more than 50 microns) 5 d 56 62 58 has a high resistance to detrition. A method of obtaining such l e o m a crystal structure comprises the addition of from 0.01 to 0.1 percent by weight of an alkaline earth metaborate to the start- [56] References cued ing mixture of the metal oxides to be sintered, e.g. NiO, ZnO

UNITED STATES PATENTS and F9203- 3,l 17,934 l/l964 Linares ..252/62.63 3 Claims, 1 Drawing Figure PATiNTEnJunzo 1972 INVENTORJ AGENT The invention relates to a method of manufacturing a sintered, oxidic, ferromagnetic body consisting of interlocking crystals having an average grain size of more than 50 microns.

It is known to use a sintered oxidic ferromagnetic body consisting of ferrite" as a core for a magnetic head for reading, writing and erasing magnetic recordings. A ferrite is to be understood to mean herein a soft-magnetic solid consisting of cubic crystals having a chemical composition according to the formula (MO. Fe O (ZnO. v Fe O wherein M is a bivalent metal, for example, Ni (nickel) or Mn (manganese), while 0.8 1.50.8 v 1.50 2 1.

Especially in the last few years a clearly increasing tendency towards the use of increasing tape speeds has manifested itself. This means that more and more stringent requirements have to be imposed upon the operative face of a magnetic head, as regards the mechanical resistance to wear. It notably becomes increasingly difficult to prevent that during operation of the magnetic head, crystals are torn out of the operative face. In regular crystal structures the pull out of a crystal takes place comparatively easily along the boundary surface of a grain. Once in such a crystal structure one crystal has been torn out, the surrounding crystals miss their original support and there is a fair chance that more and more crystals will pull out. The properties of the magnetic head which is manufactured from such a material will more and more deteriorate as a result of this pull out of crystals out of the operative face as a result of the scouring effect of the record carrier, particularly when the pull out is in the proximity of the operative gap.

In German Pat. specification No. 1,094,995 a sintered oxidic ferromagnetic body has been proposed consisting of interlocking ferrite crystals which are as large as possible as a core for a magnetic head which must have a large mechanical resistance to detrition. In such a crystal structure the possibility that a crystal is torn out is much smaller since the large interlocking crystals hold" each other. It is proposed in particular in this said German patent specification to use a ferrite monocrystal as a core for a magnetic head. The method of manufacturing ferrite monocrystals, however, is expensive also as a result of the low production output. Moreover, it is difficult to obtain stress-free monocrystals (stress freedom is an imperative requirement in this connection). In addition it has been proposed to select pieces having the-desirable structure from a melted ferrite (a ferrite formed by chemical reaction in the liquid phase for which purpose temperatures of 1,600" C. and higher are required). Technically, however, this method is not very attractive while in addition the magnetic properties of melted ferrite are not as good as a result of oxygen loss occurring at the required high temperatures of production. Another method of manufacturing a polycrystalline ferrite body consisting of crystals which are as large as possible is that of the so-called hot pressing. However, this method is very complicated and hence expensive.

The method according to the invention enables in a simple and cheap manner the manufacture of a sintered oxidic ferromagnetic body which fulfils the above-mentioned requirements. It is characterized in that from 0.01 to 0.10 percent by weight (preferably from 0.0l to 0.05 percent by weight) of an alkaline earth metaborate is added to the starting mixture of the materials to be sintered during manufacturing according to the normal manufacturing method. This normal method involves that a pre-compressed body consisting of a finely divided mixture of the starting substances (oxides, or compounds convertible into oxides at the temperature of the sintered process) is sintered in a gas atmosphere.

According to the invention calcium metaborate is preferably added to the starting mixture.

The invention also relates to a sintered oxidic ferromagnetic body, manufactured by using the method according to the invention.

The composition of the starting material is preferably chosen to be so that the relative quantities of the metal oxides constructing the ferrite in the sintered oxidic ferromagnetic body manufactured by using the method according to the invention expressed in mol. percent lie between the following limits:

5-35 mol percent NiO. l5-45 mol percent ZnO. 48-50 mol percent Fe,O;,.

A ferrite body having this composition and manufactured in the manner described is excellently suitable owing to its excellent detn'tion properties to serve as a core for a magnetic head, especially when high tape speeds are required. However, in manufacturing ferrite bodies which are non-magnetizable at room temperature and which can be used, for example, as spacing members and reinforcing members, respectively, between the magnetizable circuits in magnetic heads, the method according to the invention can advantageously be applied. In this connection is is to be noted that the method according to the invention can successfully be used in manufacturing all those sintered ceramic bodies which are used in places where they are exposed to agressive mechanical forces. Such sintered ceramic bodies, manufactured while using the method according to the invention, are characterized in that they contain from 0.01 to 0.1 percent by weight of alkaline earth borate.

The invention will now be described with reference to the following example with associated copy of a microphotograph.

EXAMPLE 0.05 percent by weight of calcium metaborate is added to a finely divided mixture consisting of 12.5 percent by weight of NiO, 23.0 percent by weight of ZnO and 64.5 percent by weight of Fe O This mixture is prefired for 1 hour at a temperature of 850 C. in an oxygen-containing atmosphere. The prefired product is then ground and compressed, preferably isostatically. The compressed product is then fired for 20 hours at a temperature of l,200 C. in pure oxygen followed by cooling to room temperature. The ferrite body obtained in this manner is found to consist of interlocked crystals having an average grain size of more than 50 microns, as is shown in the copy of the microphotograph.

It should be noted that it has been found to be of essential importance that the quantity of alkaline earth metaborate to be added to the starting mixture lies between the said limits. When, for example, 0.2 percent by weight of an alkaline earth metaborate is added, undesirable agglomerations of pores are formed. It has also been found to be of an essential importance that the said quantity of alkaline earth metaborate is added to the starting mixture. When it is added at a later stage, a crystal structure having large holes is formed.

What is claimed is:

1. In the method of manufacturing a sintered oxidic ferromagnetic body consisting of interlocking crystals having an average grain size of more than 50 microns, in which a mixture of 5-35 mol% Ni0, about 15-45 mol% ZnO, and about 48 to 50 mol percent Fe 0 is formed which is compacted into bodies which are sintered at a temperature of about l,200 C. and cooled, the step of adding to said mixture about 0.01 to 0.10 percent of an alkaline earth metaborate.

2. A method as claimed in claim 1, in which from 0.01 to 0.05 percent by weight of an alkaline earth metaborate is added to the starting mixture of the materials to be sintered.

3. A method as claimed in claim 1 in which alkaline earth metaborate is added in the form of calcium metaborate. 

2. A method as claimed in claim 1, in which from 0.01 to 0.05 percent by weight of an alkaline earth metaborate is added to the starting mixture of the materials to be sintered.
 3. A method as claimed in claim 1 in which alkaline earth metaborate is added in the form of calcium metaborate. 